Preparation of ultra-fine metallic powders. A study of the structural transformations and of the sintering behaviour

Abstract

Ultra-fine metallic powders—iron, nickel, cobalt, chromium, FeCo, FeNi—were prepared by levitation melting in a cryogenic liquid (argon or nitrogen). Most of the particles were spherical single crystals. The particle size varied from about 10 nm to 200 nm, the mean diameter being 30 nm or 70 nm depending on the experimental conditions. The austenite-martensite transformation in ultra-fine FeNi powders (25 wt.% Ni, 30 wt.% Ni and 35 wt.% Ni) was investigated. It is suggested that either martensite or austenite single particles might be formed directly from the metallic vapours of the calefaction layer. Austenite stabilization would result from the increasing difficulty of plastic accomodation. The dilatometric study of the sintering behaviour showed that shrinkage is much larger for ultra-fine powders than for the classical powder (5 μm).